1. Field of the Invention
The present invention relates to a radiation detector with a pixel electrode using gas amplification and a method for manufacturing the radiation detector.
2. Background of the Invention
Conventionally, a pixel radiation detector would be employed as a radiation detector using gas amplification. In the conventional pixel radiation detector, a strip cathode electrode is formed on the main surface of a double-sided printed substrate and a strip anode electrode is formed on the rear surface of the same printed substrate while openings are formed in the strip cathode electrode at a constant pitch and columnar anode electrodes are protruded at the centers of the respective openings, the columnar electrodes being electrically connected with the strip anode electrode. In this case, the columnar electrodes constitute the pixel electrode of the pixel radiation detector.
Here, the radiation detector is disposed in an atmosphere containing He gas and methane gas. Then, a voltage of 600 V is applied between the pixel electrode and the strip cathode electrode, for example.
In the radiation detector, when a given radiation is input into the detector, the gas is ionized to generate electrons. Since a large voltage is applied between the strip cathode electrode and the pixel electrode such that an electric field with a high intensity is generated due to the configuration of point electrode of the pixel electrode (i.e., the shape anisotropy of the pixel electrode), electron avalanche amplification is brought about. On the other hand, the positive ions generated by the electron avalanche amplification are drifted to the corresponding approximate edges of the strip cathode electrode from the pixel electrode (i.e., the columnar electrodes).
As a result, the holes and electrons are charged to the corresponding strip cathode electrode and pixel electrode (columnar electrodes). Therefore, when the charging positions of the strip cathode electrode and the pixel electrode (i.e., columnar electrodes) are detected, the input position of the radiation in the detector can be specified and thus, the radiation can be detected (Reference 1).    [Reference 1] JP-A 2002-006047
In the radiation detector, as the voltage to be applied to the pixel electrode is increased, the intensity of the electric field to be generated is increased so that the electron avalanche amplification is brought about remarkably. In this case, since the amounts of charge at the strip cathode electrode and the pixel electrode (i.e., columnar electrodes) are increased, the detection sensitivity of the radiation can be also increased. On the other hand, if a large voltage is applied to the pixel electrode, particularly, arc discharge occurs between the pixel electrode and the edges exposing to the corresponding openings of the strip cathode electrode so as to damage the strip cathode electrode and the pixel electrode.
In this way, even though the voltage to be applied to the pixel electrode is increased in order to enhance the detection sensitivity of the radiation detector, the arc discharge occurs so that the radiation detector can not be functioned than the detection sensitivity of the radiation detector is enhanced.
In contrast, when the voltage to be applied to the pixel electrode is decreased, the arc discharge is unlikely to occur while the electron avalanche amplification is reduced so that the detection sensitivity of the radiation detector for the radiation is decreased.
In this point of view, such an attempt is made as narrowing the pixel electrode (i.e., the columnar electrodes) and enhancing the intensity of the electric field to be generated instead of controlling the voltage to be applied to the pixel electrode. However, since the pixel electrode is configured such that the through holes formed in the printed substrate are embedded by means of via-fill plating, it is required to narrow the through holes in order to narrow the pixel electrode.
In contrast, when the through holes are narrowed, the via-fill plating can not be uniformly carried out for the through holes so that the pixel electrode can not be also formed uniformly. In this case, anomalous discharge and dielectric breakdown disadvantageously occur. Therefore, the narrowing of the pixel electrode is restricted spontaneously in dependence on the forming method.
As a result, as of now, the voltage to be applied to the pixel electrode can not be sufficiently developed while the narrowing of the pixel electrode can not be sufficiently realized, so that the detection sensitivity of the pixel radiation detector can not be sufficiently enhanced.